This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Obesity is a strong risk factor for developing type 2 diabetes (T2DM), but the reasons for this are not fully understood. In particular, it is not known why some obese people develop T2DM while other obese individuals do not. This study tests whether differences in fat cells (adipocytes) are to blame. Even in adults, fat cells are constantly being formed to replace old fat cells and to respond to the body's need to store excess energy. The ability to form new fat cells may be diminished in some individuals, leading to larger fat cells. These large fat cells secrete hormones that may increase risk for T2DM. This study tests whether fat cells from obese insulin resistant subjects (who are at risk for developing T2DM) form at a slower rate than those from insulin sensitive subjects (who are at lower risk for developing T2DM). To address this question we will recruit and study two groups of obese subjects, selected to be similar in age, gender and degree of obesity. One group of subjects will be obese and insulin resistant (the OIR group), while the other will be comparably obese, but insulin sensitive (OIS). Subjects will undergo a series of studies to characterize their metabolism including measurement of body fat by DXA scanning, oral glucose tolerance (a test used to diagnose diabetes) and measurement of insulin sensitivity in response to an infusion of insulin (a research study used to classify patients into the OIR and OIS groups). Small samples of fat (from just under the skin of the belly and the buttocks) will obtained using a needle on two occasions over 12 weeks. During these 12 weeks, subjects will drink a small amount of water that contains a non-radioactive label. This labeled water will allow us to measure the rate of growth of new fat cells in the body. We will also look at the rate of growth of fat cells obtained from these biopsies in the laboratory. The results of this study may tell us more about why certain obese people develop diabetes and why others do not. This might lead to new ways to prevent or treat T2DM.